Reginald Chance admitted to gun and drug possession in an April statement before being offered a no-jail plea deal. Photo: G.N. Miller

Officials tripped over themselves Monday to explain how a thug who copped to gun and drug possession in the spring was still roaming around free last week — when he sparked the biker-mob beatdown of a Manhattan driver.

Judge Suzanne MondoPhoto: Jennifer S. Altman

Reginald Chance, 37, was busted Saturday for allegedly smashing the window of dad Alexian Lien’s Range Rover with his helmet before other riders beat the Columbia grad in the vicious road-rage attack.

Chance already had 21 busts under his belt when he was arrested in April over a cocaine sale involving an undercover cop in Brooklyn, records show.

Although they had no warrant, investigators went to search the nearby apartment of Chance’s girlfriend — and found the door open and the girlfriend let them in, sources said.Inside, they found Chance, a .22-caliber handgun, cocaine and pot, all of which Chance admitted were his both in a handwritten statement and before a grand jury, according to records and sources.

Chance could have faced felony gun and drug raps and landed up to 25 years behind bars. But the grand jury didn’t indict him on the gun charge and the drug charges were lowered to a misdemeanor pot possession in a deal with prosecutors that was signed off on by Judge Suzanne Mondo.

Law-enforcement sources noted many factors went into the decision to give Chance a conditional release, including that he had only three misdemeanor convictions and no history of physical violence.

“Taking into account that the defendant was not the hand-to-hand who gave the informant the drugs, the defendant did . . . do community service, his past record was only misdemeanors, no felonies, and the last of which was 6 years old, given those facts, it’s not a an unreasonable plea deal,” the source said.

Of course, “This was before we knew what he did with the helmet,” the source added.